1. Field of the Invention
The present invention relates to method for injection-molding a plastic substrate used in high-density information recording carriers or media such as an optical disk including ROM disks such as CD (compact disk), VD (video disk), DRAW disks, E-DRAW disks, optical cards or the like.
2. Description of the Related Art
The substrates used in high-density information recording carriers or media are generally made of glass, metal, ceramics or plastic materials. The plastic substrates are preferable because of their lightweight, high strength and high productivity.
The plastic substrate for optical disks is a thin flat plastic disks having a thickness of about 1.2 to 1.5 mm and an outer diameter of less than about 300 mm. Such thin flat disks are not easy to be produced by injection molding machines because resin materials such as acryl resins or polycarbonate resins of which the plastic substrates are made have very poor melt-flow properties or "flowability". In other words, it is very difficult to produce such a plastic substrate that satisfies two requirements in an optical property such as low birefringence and in a molding property such as high pattern-transferring property.
The high pattern-transferring property means such an ability that a surface replica patter, in other words, a concave-convex pits pattern on a surface of the stamper held on the mold is transferred precisely onto a plastic substrate molded, and is one of the most important factors that are required to produce plastic disks used for the above-mentioned type applications.
FIGS. 1 and 2 illustrate two typical structures of molds which are usually used in the conventional injection molding machines for producing the plastic substrate for optical disks.
The mold assembly shown in FIG. 1 has a pair of mold halves comprising a movable mold half 1 and a stationary mold half 2 which define a molding cavity 3. On one of the mold halves 1 and 2 (in the case shown in FIG. 1, on the movable mold half 1), a so-called stamper 4 is held by a stamper holder 5. The stamper 4 is a replica which is usually manufactured by a mastering machine. The stamper 4 or a replica has a concave-convex patterned surface which is composed of information pits and tracking grooves having dimensions of sub-micron order. On another mold half (in the case of FIG. 1, on the stationary mold half 2), an outer ring 6 is secured so that at a radially outer periphery of the molding cavity is defined by the outer ring 6. Molten resin is injected into the molding cavity 3 through a sprue 7.
FIG. 2 illustrates another type injection mold assembly. In FIG. 2, a dash (') is put to same reference numbers for members which correspond to FIG. 1. This type mold assembly which is called generally "inlay" type has no outer ring such as the ring 6 of FIG. 1 but has such a structure that an tapered inner side surface 10 of a stamper holder 5' will contact with a complementary surface 9 of the stationary mold half 2', so that an outer periphery of the molding cavity 3' is defined by the tapered side surface 10.
The stationary mold half 2 or 2' is secured to a stationary platen (not shown), while the movable mold half 1 or 1' is secured to a movable platen (not shown) of an injection molding machine.
In operation, after the mold halves 1 or 1' and 2 or 2' are mated to each other to close the molding cavity 3 or 3', molten resin is injected into the molding cavity 3 or 3' through the sprue 7 or 7'.
After a molded plastic disk is left to be cooled or solidified for a predetermined time duration, the mold is opened. In this mold-opening operation, the movable mold half is displaced or retracted from the stationary mold half by de-actuating a mold clamping mechanism (not shown) to open the mold halves. Then, air is blown through air injection holes arranged in the neighborhood of a center of the stationary mold half to release the molded plastic substrate from the stationary mold half at first. Therefore, in the initial stage of the mold-opening operation, the molded plastic substrate is held on or sticks to a surface of the stamper 4 which is supported on the movable mold half.
Then, after the mold halves are opened completely, air is blown through another air injection holes arranged in the neighborhood of a center of the movable mold half to release the molded plastic substrate from the stationary mold half secondly. Then, ejector rods (not shown) are actuated mechanically to eject the molded plastic substrate out of the mold.
The optical disks are classified into two categories of a preformatted type and a post-formatting type.
In the case of post-formatting type, only trucking guide grooves (continuous grooves) are engraved or molded on a surface of the plastic substate. On the guide grooves of the plastic substrate, a recording layer is deposited in a next recording layer forming stage. Formatting signals are cut or recorded on the recording layer in a so-called formatting stage in an optical disk driving unit. Therefore, a problem to be solved in the process for molding plastic substrates used for such post-formatting type optical disks is how to transfer precisely the embossed concave-convex trucking grooves from the stamper onto the plastic substrate surface.
The pre-formatted type is apparently much economical than the post-formatting type because the formatting operation in the optical disk driving unit can be eliminated. In fact, in this case, both of format signals and trucking grooves must be engraved or molded simultaneously on the surface of the plastic substrate in the molding stage. Such pre-formatted type optical disks which are the main trend in the field of optical recording media can be realized by an ultra-fine molding process as well as the recent development in the mastering technique and in the optical disk driving unit. In the case of preformatted type optical disks, however, it arises still another problem, because the format signals consist of a series of concave-convex pits each having a predetermined length which must be transferred much precisely than the trucking groove each consisting of a simple spiral or continuous groove. Generally, such fine pre-format signal pits are cut or engraved on a surface of the stamper by means of the mastering machine.
When a plastic substrate is molded according to the conventional injection mold process in the conventional mold assembly in which the stamper having such pre-format signal pits on its side surface is held, it is often observed such a defect that the transferred pits have deformed or distorted configuration by a phenomenon of "flow of pits" towards the enter of the disk, in other words, each concave-convex pit is not transferred precisely from the stamper to the plastic substrate but is distorted or becomes "dull" on the plastic substrate obtained.
Such plastic substrate of which pre-format signal pits are "dull" or distorted can not be used for application of optical disks. In fact, a reflection light from a recording layer deposited on such deformed plastic substrate deviates from a predetermined intensity, resulting in that high performance in recording and play-back characteristics of the optical disk can not be obtained.
The present inventors found such a fact that the above-mentioned phenomenon of "flow of pits" occur just after the mold is opened but before the molded plastic substrate is separated completely from the stamper, and completed the present invention.
Therefore, an object of the present invention is to provide a method to overcome the problems in prior arts such as poor quality of the pit-transfer from the stamper to the plastic substrate caused by the phenomenon of "flow of pits" and also to provide an improved mold assembly used to realized the method according to the present invention.